Multi-band vertical antenna.
The proposed vertical antenna (Fig. 1) was designed as a compromise for a satisfactory quality of work on all of the major Amateur LW bands.
Vibrator universal antenna is made of aluminum tubing with a diameter of 40 to 50 mm In the upper part is the filter tube that is configured to average the frequency range of 40 MHz (7,05 MHz). In the range of 80 m, the filter is inductive reactance and "extends" the vibrator to the electrical length L/4. In the range of 40 m of the filter tube is "off" the upper part of the vibrator, whereby the electrical length of the antenna is also equal to its L/4. In the range of 20 m, the resistance of the filter is capacitive in nature and its length becomes equal to 3L/4.
In the ranges 21I 28 MHz filter-tube shortens vibrator electrically to the length 5L/4 and 7L/4, respectively.
The counterbalance system consists of 20 segments of copper wire with a diameter of 1...3 mm, connected at a single point under the vibrator. At each of the five ranges used four PA counterweight, for the range of 80 m length of 20.8 m, 40 m and 10.4 m, 20 m - 5.2 m 15 m - 3,49 m, 10 m - 2,62 m. In Fig. 2 shows the arrangement of counterweights in one quadrant.
The antenna is fed by a 50 - or 75-Ohm coaxial cable with a length equal to a half wavelength in the range of 80 m. This simplifies the coordination of the antenna with feeder, as an active antenna impedance changes significantly when moving from one band to another.
Described antenna installed on the roof of a four-storey reinforced concrete prefabricated house. The best results were achieved in the range of 80, 40 and 20 meters the SWR on any of the ranges does not exceed 2.
If the antenna will be placed on the ground or close to it, should be grounded connection point balances (their length in this case should be about 0.4 L), and the balances to dig into the soil to a depth of 40...70 cm.
Author: M. Chirkov (UL7GCC), Alma-ATA; Publication: N. Bolshakov, rf.atnn.ru